Aircraft control apparatus



Oct. 30, 1945. 5 G, |5$ER$TEDT 2,387,795

AIRCRAFT CONTROL APPARATUS Filed March 26, 1943 07u55 To v 3 I um I 9P05/770m 9 g 961 )fl/0175 l? Inventor Cttorneg Patented Oct. 30, 1945AIRCRAFT CONTRGL APPARATUS Siegfried G. Isserstedt, Toronto, Ontario,Canada, assignor to Minneapolis-Honeywell Regulator Company, of DelawareMinneapolis, Minn., a corporation Application March 26, 1943, Serial No.480,674

7 Claims.

This present invention relates to control apparatus for aircraft, moreparticularly to control apparatus for automatically positioning acontrol surface, such as a rudder, aileron, or elevator.

It has previously been proposed to operate a control surface on anaircraft under the control of a sensitive device responsive to acondition which indicates the need for operation of the control surface.For example, it has been proposed to control the movements of the rudderof an aircraft in response to :the deflections of a direction responsivedevice such as a directional gyroscope or a compass. 'I'he presentinvention is applicable to systems of that type.

When an aircraft control surface, such as a rudder, aileron, orelevator, is deflected from its normal position, it encounters airresistance which tends 4t0 move it laterally, back towards its normalposition. If it is held in its deflected position by means of a cable orother device under the control of the pilot or under suitable automaticcontrol, the force due to air resistance is transmitted to the aircraftitself, causing it to turn, if the control surface concerned is arudder, to tilt if the control surface is an aileron, or to climb ordive if the control surface is an elevator.

It will be readily understood that the air resistance encountered by thecontrol surface when deflected from its normal position, varies inaccordance with the speed of the aircraft and in accordance with thedensity of the air through which the craft is moving. When the aircraftis moving at a high speed, the air resistance encountered by the controlsurface is much greater than when the aircraft is moving at a low speed.Similarly, when the aircraft is traveling fairly close to the ground,the air is more dense than at high altitudes, and the air resistanceencoun- 1 tered by the control surface is correspondingly greater.

It has been found that the air resistance encountered by a controlsurface does not vary directly with the speed of the aircraft and theair density, but that it varies as a non-linear function of theseconditions.

t is therefore an object of this invention to provide, in an automaticsystem for positioning a, control surface on an aircraft, means forcompensating the operation of the system in accordance with a non-linearfunction of the air speed and altitude of the aircraft.

Another object of the present invention is to provide an improvedautomatic control system for positioning a control surface on anaircraft, of the type wherein the control surface movement, under anygiven conditions, has a definite ratio with respect to the movements ofthe gyroscope or other sensitive controlling device.

A further object is to provide, in a system of the type described, meansfor varying the ratio between the control surface movement and themovement of the controlling device in accordance with Ithe air speed andaltitude of the aircraft. A still further object is to vary that ratioas a non-linear function of the altitude and air speed.

Other objects and advantages of the present invention will becomeapparent from a consideration of the attached specification, claims, anddrawing, in which: l

The single figure represents, somewhat diagrammatically, an automaticelectrical rudder control system embodying my invention.

In the drawing is shown a system in which cables I0 are attached to anaircraft rudder (not shown) to control the position thereof. The cableI0 passes over a pulley II which is driven by a motor generallyindicated as I2 through a gear train schematically shown at I3. Themotor I2 comprises an armature I4 and a pair of field windings I5 andI6, which, when selectively energized, cooperate with the armature I4 tocause rotation thereof in opposite directions.

The selective energization of the field windings I5 and I6 is controlledby a pair of electrical discharge devices I'I and I8, which are in turnselectively rendered conductive in accordance with the sense ofunbalance of an electrical bridge circuit generally indicated at 20. Thebridge circuit 20 includes a control potentiometer 2l, a rebalancingpotentiometer 22, and a compensating rheostat 23.

The control potentiometer 2i comprises a slide- Wire resistance 24, anda slider 25 movable along the resistance 24. The slider 25 is mounted ona shaft 2S, which is rotated by a directional gyroscope generallyindicated at 2l in accordance with the deviation of the aircraft from apredetermined course.

The followup potentiometer 22 comprises a slidewire resistance 30 and aslider 3i movable along the resistance 30. The slider 3l is attached toa shaft 32, which is driven by the motor I2 through the gear train l 3.

The compensating rheostat 23 comprises a tapered slidewire resistance33, and a slider 34 movable along the resistance 33. The slider 34 ispivotally supported at 35, and is rotated about its pivot 35 inaccordance with the translation of a connecting rod 36 by a pressureresponsive control device generally indicated at 31.

The control device 31 comprises a base 40, which may be attached to anysuitable support, and an outer casing 5I carried by the base 40. Insidethe Casing 4I and spaced substantially from the sides thereof is aexible bellows 52. One end of the bellows l2 is attached, by anysuitable means, to the base 56. The other end of the bellows 82 isclosed by a suitable wall portion 43. The end of connecting rod 36opposite the slider 38 is attached to the wall portion B3 by a suitableconnection.

The space enclosed by the base 60, the casing 8|, and the bellows 52forms a chamber 55. The rod 36 passes through an aperture in the basefill which is closed by a suitable sealing bellows 55. One end of thebellows 45 is suitably attached to the sides of the aperture in the base58, and the other end is provided with a Wall portion which is attachedto the rod 36. All the joints between the bellows d2 and 55, base 5D,and rod 36 may be made, for example, by soldering. The space enclosed bythe bellows l2 and 55 and the base 56 forms a chamber 56. The chamber 46may be evacuated, or it may be filled or partially filled with a fluidhaving such characteristics that a substantially constant pressure ismaintained therein. f

The chamber 55, on the other hand, is connected to a tube 51 whoseopposite end is exposed to the air stream passing the aircraft in whichthe system is used.` The pressure to which the outer end of tube 81 isexposed, and which is communicated to the chamber 55, is sometimesreferred to as the dynamic pressure. This pressure includes twoindependently varying components, one of which is the atmosphericpressure, which changes with the altitude of the aircraft, and the otherof which is the pressure due to the forward motion of the aircraftthrough the air. It should therefore be apparent that as the altitudedecreases or the airspeed increases, the dynamic pressure increases,moving rod 36 and slider 38 to the left. Conversely, an increase inaltitude or a decrease in airspeed causes movement of rod 36 and slider35 to the right.

The bridge circuit 28 includes input terminals 58 and 5I, and outputterminals 52 and 53, which are interconnected by the conventional fourarms of a Wheatstone bridge circuit. The upper left arm of bridgecircuit 28, as it appears in the drawing, connects input terminal 58with output terminal 52, and may be traced from input terminal 58through a fixed resistance 56, a conductor 55, a portion of slidewireresistance 24, and

slider 25 to output terminal 52. The upper right l arm of bridge circuit28 connects input terminal 5I with output terminal 52,` and may betraced from input terminal 5I through a xed resistance 56, a conductor51, a portion of slidewire resistance 2li, and slider 25 to outputterminal 52. The lower left arm of bridge circuit 28 connects inputterminal 58 with output terminal 53 and may be traced from inputterminal 58 through a conductor 58, a portion of slidewire resistance38, and slider 3| to output terminal 53. The lower right arm of bridgecircuit 20 connects input terminal 5I with output terminal 53 and may betraced from input terminal 5I through a conductor '60, a portion ofslidewire resistance 38, and slider 3l to output terminal 53.

While the output terminals 52 and 53 are shown as mounted on sliders 25and 3|, respectively, it

should be understood that each output terminal is electrically andphysically identical with its associated slider. The application ofseparate reference characters to the Output terminals is merely forconvenience in describing the operation of the system.

The input terminals 50 and 5I are connected by conductors 6I and 62,respectively, to a suitable power supply, such as the secondary winding63 of the transformer 65, having a primary winding 65, which may beconnected to any suitable source of alternating electrical energy,represented in the drawing by supply lines 93 and 94.

The left hand terminal of rheostat resistance 33 is connected to theleft hand terminal of slidewire resistance 24 by means of a conductor68, and the rheostat slider 34 is connected to the right hand terminalof slidewire resistance 24 by means of a conductor 61.

The output terminals 52 an 53 of bridge circuit 28 are connected by abridge output circuit, which may be traced from output terminal '52through a conductor 18, a resistance 1I, a resistance 12, and aconductor 13 to output terminal 53.

The discharge device I1 is of the gas-filled type and is provided withan anode 15, a control electrode 15, and a cathode 16. The dischargedevice I8 is also of the gas-filled type, and is provided with an anode11, a control electrode 18, and a cathode 19.

The discharge device I1 has an input circuit which may be traced fromthe control electrode 15 through a protective resistance 88, a conductor8l, resistance 1I, a conductor 82, a biasing battery 83, and conductors8B and 85 to cathode 16. The discharge device I8 has a correspondinginput circuit which may be traced from control electrode 18, through aprotective resistance 86, a conductor 81, resistance 12, conductor 82,.biasing battery 83, and conductors 8d and 88 to cathode 19.

The discharge devices I1 and I8 have output circuits which are suppliedwith electrical energy from a secondary winding 90 of a transformer SI,having a primary winding 92, which may be connected to any suitablesource of alternating electrical energy. This source should be of thesame frequency and phase as that to which the transformer primaryWinding 65 is connected, This connection is indicated by the fact thatboth transformer primary windings are connected to supply lines 93 and95.

The output circuit of discharge device I1 may be traced from the lefthand terminal of secondary winding 30 through motor armature I4, Iieldwinding i6, a conductor 95, anode 14, cathode 16, and conductors and 96to the right hand terminal of secondary winding 90. The disc hargedevice I8 is provided with a corresponding output circuit which may .betraced from the left hand terminal of secondary winding 98 through motorarmature I5, field winding I5, a conductor 91, anode 11, cathode 59, andconductors 88 and 96 to the right hand terminal 0f transformer winding9i?.

The cathodes 16 and 19 may be supplied with electrical energy forheating purposes from any suitable source (not shown) through theconductors 85, 98, and 88, 99, respectively.

The gaseous 'discharge devices l1 and I8 have the characteristic, usualin such devices, that when a discharge therethrough is initiated, thedischarge continues until the output circuit is externally interrupted.Although the control electrode of such a device may control theinitiation of such a discharge, the control electrode has no furthercontrol over the conductivity oi' the device after the discharge hasbeen initiated. Since the output circuits of the discharge devices l1and i8 are energized with alternating electrical energy, these devicesmay be conductive only during those half cycles when the left end ofsec.. ondary winding 90 (which is connected to anodes i4 and Tl) ispositive with respect to the right end (which is connected to cathodes'i6 and T19). These half Acycles will be hereinafter referred to as thepositive half cycles. During the opposite, or negative half cycles, theelectrical energy applied to the anodes and cathodes of the devices I?and I8 is of a ypolarity such that any current iiow therethrough isprevented. Hence a discharge initiated during any positive half cycle isstopped at the termination of that half cycle. ln the present circuit,the biasing battery 83 is so chosen as to maintain the controlelectrodes 18 and i of the discharge devices l1 and i8 norinaliy at apotential sufficiently negative with respect to their associatedcathodes that no dis charge is initiated in the discharge devices il'and i8 during the positive helf cycles. The de vices il and i8 aretherefore said to he biased to cut-off by the battery 83.

Operation First consider the operation of the control syse tern with theslider 34 stationary. After such operation has been described, theeffect of :movement of the slider 3ft on the operation of the systemwill be further considered.

When the control potentiometer 2i and the follow-up potentiometer 22 arein the positions shown in the drawing, both the sliders 25 and Si are ata potential corresponding to the median potential of the secondarywinding 63. Since both the output terminals E2 and 53 are at the sainepotential, no current flows in the bridge output circuit connectingthose terminals. There is therefore no potential drop across either. ofthe resistances il and '12. As previously mentioned, the dischargedevices il and i8 are biased to out o'f the battery 83, and thereforeboth devices remain non-conductive. The motor i4 remains stationary, andthe rudder remains in its normal or neutral position.

Let it now be considered that the aircraft in which this system is useddeviates from its predetermined course in such a direction that thegyroscope 27 rotates the shaft 26 so as to move slider 25 to the leftalong resistance 2d.

Considering the condition of the bridge circuit when the slider 25 hasbeen moved to the left from the position shown in the drawing, it willbe seen that the slider 25 and hence the output 'terminal 52 hasattained a position which is electrically closer to the left end ofsecondary winding 63 than to its right end, and hence, during thepositive half cycles of the power supply,

is positive with respect to output terminal 53. Therefore a currentiiows in the output circuit connecting terminals 52 and 53 in adirection from terminai 52 to terminal 53. This current, rlowing throughresistances Il and 72, sets up a potential difference across each ofthese resistances of such a polarity that their upper terminals arepositive with respect -to their lower terminals. In the input circuit ofthe discharge device I8, this potential drop across resistance 'i2 is ofthe same polarity as that of battery 83,

more negative, so that the discharge device I8 remains nonconductive. Inthe input circuit ci the discharge device il, however, this potentialdrop is of a polarity opposite to that of battery 83. This positivepotential therefore partially counteracts the eect of biasing batterySi, and carries the control electrode 'l5 to a potential which is morepositive than the cut-ofi potential of the device il. A discharge istherefore initiated through the device il, and is maintained throughoutthe half cycle during which anode 'i4 is positive with respect tocathode i8. During this discharge, current flows through the outputcircuit of discharge device il, previously traced, thereby energizingarmature iii and field. winding I6 of motor l2. Such energization ofmotor I2 causes its rotation in a direction so as to drive the rudderthrough gear train i3, pulley il and cables il! to restore the aircraftto its predetermined course. .at the time, the slider 3l of rebalanciigpotentiometer 22 is driven by the motor i2 through gear train i3 andshaft i2 so as to move toward the left end of resistance 3U.' As long asthe bridge 2i! remains unbalanced in the same sense, that is, so thatthe output terminal 52 is positive with respect to output termihal 53,the motor i2 is energized on each positive half cycle so as to drive theslider Si iurther to the left and at the saline time introduce acorrecting deection of the rudder. When the slider 3i has moved farenough to the left so that output terminal 53 is at the same potentialas output terminal 52, the bridge circuit 2i) is again balanced and thedischarge device il remains non-conductive, the motor l2 beingdeenergized and the rudder remaining in its deflected position.

As the aircraft returns to its predetermined course in accordance withthe corrective deflection of the rudder, the gyroscope 2l moves theslider 25 back toward its center position. This movement of slider 25connects output terminal 52 to a point on resistance 2d which is morenegative, during the positive half cycles of the power supply, than thepoint on resistance 3B to which output terminal 53 is connected. Currenttherefore flows in the output circuit connecting terminals 52 and 53 ina direction from terminal 53 to terminal 52. This current, flowingthrough resistances l2 and "li, sets up a potential difference acrosseach o1 these resistances in such a direction that their lower terminalsare positive with respect to their upper terminals. In the input circuitof the discharge device il, this potential drop across resistance 'll isof the same polarity as the potential of the biasing battery 33, and thecontrol electrode l5 is therefore made more negative, so that thedischarge device l'i remains non-conductive. In the input circuit of thedischarge device it, however, the potentia1 drop across resistance 'i2is in the opposite sense to the potential of the biasing battery 33. Thecontrol electrode 'i8 is therefore supplied with a potential morepositive than the negative value which maintains the device i8 cut offand a discharge is initiated through the device i8. This dischargepersists throughout the positive half cycle, and while the dischargeexists, a current iiows through motor armature M and field winding i5.Energization of motor l2 in this manner causes it to drive the rudder inthe opposite direction, back toward its normal position, and

` at the same time the slider 3i is driven to the right along resistance30. This motion of the rudand the control electrode 18 is therefore made75 der and the slider 3l continues in the same direction as long as thebridge is unbalanced in'a sense which makes output terminal 53 positivewith respect -to output terminal 52. As soon as the aircraft hasregained its predetermined course, and the rudder is returned Ato itsnormal or neutral position, bridge circuit 2t) is again balanced and themotor i2 is stopped.

Since the rudder and the slider 3l of follow-up potentiometer 32 aremechanically connected, it will be readily understood that the movementsof slider 3l are proportional to those of the rudder. From the foregoingdescription of the operation of the system, it should be also apparentthat, for each movement of .the slider along resistance 2d, the systemproduces a corresponding movement of slider 3l along resistance 30. Whenthe slider 25 moves to a point along resistance 24, having a potentialdifferent from that of the point at which slider 3l engages resistance30,

the system causes a follow-up movement of slider 3l to a point havingthe same potential as that to which the slider 25 has been moved. Itshould therefore be apparent that the ratio between a given movement ofslider 25 and the following movement of slider 3| is dependent on theratio between the voltage drop per unit length along slider 24 and thevoltage drop per unit length along resistance 30. Since the voltagegradient along the resistances 24 and 30 is substantially constantthroughout .their length, this ratio between the movement of the controlslider 25 and the rebalancing slider 3l is constant throughout the rangeof such movement.

Now consider the effect of the compensating rheostat 23 on the system.The total potential supplied by the secondary winding 63 is divided, inthe two upper arms of the bridge circuit 2l), between voltage dropsacross the fixed resistances 54 and 56 and a voltage drop across theresistance 24. When the rheostat 23 is operated, the voltage drop acrossresistance 2d is changed. Since the total voltage drop remains the same,being xed by the terminal voltage of secondary Winding 63, the voltagedrop across the fixed resistances 54 and 56 is also changed but in anopof resistance 24 is reduced. Therefore the voltage drop per unitlength of resistance 24 is reduced, and a given movement of slider 25causes a smaller change potential of output terminal 52, and henceproduces a smaller following movement of slider 3|. While a definiteratio still exists between the movements of slider 25 and the followingmovement of slider 3 I, the ratio has been increased by the movement ofrheostat slider 34 to the left of the position shown in the drawing. Ina similar manner, it will be understood that movement of thev rheostatslider 34 to the right causes a decrease in the ratio between a givenmovement of slider 25 and the following movement of slider 3 I.

Since the rheostat resistance 33 is tapered, it will be seen that thechange in resistance per unit movement of slider 34 increases as theslider moves from the left to the right end of its range of travel.Hence, the total resistance connected across the terminals of resistance24 is a nonlinear function of the position of slider 34. The position ofslider 34 is controlled by the pressure responsive device 31 so thatupon a change in altitude or air speed, such that the rudder willencounter greater air resistance, the pressure in the chamber 44 isincreased. The pressure in chamber 46 is constant, and an increase inpressure in chamber 44 causes bellows 42 to collapse slightly, movingwall 43 to the left, and causing a corresponding leftward movement ofrod 36 and slider 3d. The ratio of the controlling movement of slider 25to the following movement of slider 3l, is thereby increased to reducethe deflection of the rudder in response to a given deflection of thegyroscope 2l. This decrease in rudder deflection compensates for theincrease in dynamic pressure, resulting in asubstantially constantturning effect of the rudder in response to the gyroscope, at allconditions of altitude and air speed.

On the other. hand, if the altitude increases or the airspeed decreases,resulting in a decrease in the dynamic pressure, then the pressure inchamber it decreases. The pressure in chamber 46 being constant, thebellows 42 expands, moving rod 36 and slider 34 to the right. Asexplained above, this decreases the ratio between a given movement ofslider 34 and the following movement of slider 3|. In other words, theratio between a given deflection of gyroscope 21 and the followingmovement of the rudder is decreased, so that a larger rudder deflectionis obtained to compensate for the decreased air resistance resultingfrom the decreased dynamic pressure.

While I have shown and described a preferred embodiment of my invention,other modifications thereof will readily occur to those skilled in theart, and I therefore wish my invention to be limited only by theappended claims.

I claim as my invention:

1. A system for operating a control surface on an aircraft, comprisingin combination, motor means for positioning said surface with respect tosaid aircraft. means movable in response to a condition indicative ofthe need for operation of said surface, a main controller driven by saidcondition responsive means, a follow-up controller driven by said motormeans, means including said controllers for controlling said motor meansto move said surface proportionally tothe movements of said conditionresponsive means, means for varying the ratio between a given movementof said condition responsive means and the following movement of saidsurface, and means responsive to the speed of said aircraft relative tothe surrounding air for operating said ratio varying means so that saidratio increases with increase in said speed, said ratio varying meansincluding an element so formed that as said ratio varying means isoperated by said speed responsive means to increase said ratio, theamount of change of said ratio for a unit change in said speed decreaseswith increase in speed.

2. A system for operating a control surface on an aircraft, comprisingin combination, motor means for positioning said surface with respect tosaid aircraft, means movable in response to a condition indicative ofthe need for operation of said surface, a control potentiometerincluding an impedance element and a contact engageable with a pluralityof points on said impedance element, a connection between said conditionresponsive means and said control potentiometer for causing relativemovements of said impedance element and said contact corresponding tochanges in said condition, a follow-up potentiometer including animpedance element and a.

contact engageable with a plurality of points thereon, a connectionbetween said motor means and said follow-up potentiometer for causingrelative movements of said impedance element and said contactcorresponding to changes in the position of said control surface, meansincluding said potentiometer for controlling said motor means to movesaid surface proportionally to the movements of said conditionresponsive means, means for varying the ratio between a given movementof said condition responsive means and the following movement of saidsurface comprising a variable impedance connected in parallel with oneof said potentiometer impedance elements, and means responsive to acondition indicative ofthe air resistance encountered by said surfacefor operating said variable impedance.

3. A system for operating a control surface on an aircraft comprising incombination, motor means for positioning said surface with respect tosaid aircraft, means movable in response to a condition indicative ofthe need for operation of said surface, a control potentiometerincluding an impedance element and a contact engageable with a pluralityof points on said impedance element, a connection between said conditionresponsive means and said control potentiometer for causing relativemovements of said impedance element and said contact corresponding tochanges in said condition, a follow-up potentiometer including animpedance element and a contact engageable with a plurality of pointsthereon, a connection between said motor means and said follow-uppotentiometer for causing relative movements of said impedance elementand said contact corresponding to changes in the position of saidcontrol surface, means including said potentiometers for controllingsaid motor means to move said surface proportionally to the movements ofsaid condition responsive means, means for varying the ratio between agiven movement of said condition responsive 'means and the followingmovement of said surface comprising a tapered resistance, a Contactmovable along said resistance,

and connections between said contactand one end of said resistance,respectively, and the opposite-ends of one of said potentiometerimpedance elements, and means responsive to a condition indicative ofthe air resistance encountered by said surface for moving said movablecontact along said tapered resistance in such a direction as to vary thepotential drop across said one impedance upon an increase in saidcondition at a rate which decreases with an increase in the value ofsaid condition.

fi. Apparatus for controlling the position of a control surface on anaircraft comprising, in combination, motor means for positioning saidsurface with respect to said aircraft, means movable in response to acondition indicative of the need for operation of said surface, acontrol impedance including an impedance element and a membercooperating therewith for adjusting the impedance value of saidimpedance, a connection between said condition responsive means and saidadjusting member for adjusting said impedance corresponding to changesin said condition, a iollow-up impedance including an impedance elementand a member associated therewith for adjusting said impedance, aconnection between said motor means and the adjustable member of saidfollow-up impedance for causing adjustment of said follow-up impedancecorresponding to changes in the position of said control surface, meansincluding said control and follow-up impedances for controlling saidmotor means to move said surface proportionally to the movements of saidcondition responsive means, means for varying the ratio between a givenmovement of said condition responsive means and the following movementof said surface comprising a variable impedance connected in parallelwith one of said previously named impedance elements, and meansresponsive to a condition indieative of the air resistance encounteredby said surface for operating said variable impedance.

5. Apparatus for controlling the position of a control surface on anaircraft comprising, in combination, electrical motor means forpositioning said surface with respect to said aircraft, means movable inresponse to a condition indicative of the need for operation of saidsurface, a control impedance including an impedance element and a membercooperating therewith for adjusting the impedance value of saidimpedance, a connection between said condition responsive means and saidadjusting member for adjusting said impedance corresponding to changesin said condition, a follow-up impedance including an impedance elcmentand a member associated therewith for'adjusting said impedance, aconnection between said motor means and the adjustable member of saidfollow-up impedance for causing adjustment of said follow-up impedancecorresponding to changes in the position or" said control surface, anelectronic amplifier controlling the operation of said electrical motormeans, a bridge including both said control and follow-up lmpedances,said bridge being connected to said amplifier for controlling said motormeans to move said surface proportionally to the mvements of saidcondition responsive means, means for varying the ratio between a givenmovement of said condition responsive means and the following movementof said surface comprising a variable impedance connected in parallelwith one of said previously named impedance elements, and meansresponsive to a condition indicative of the air resistance encounteredby said surface for operating said variable impedance.

6. Apparatus for controlling the position of a control surface on anaircraft comprising, in combination, motor means for positioning saidsurface with respect to said aircraft, means movable in response to theattitude of the aircraft, a control impedance including an impedanceelement and a member cooperating therewith for adjusting the impedancevalue of said impedance, a connection between said attitude responsivemeans and said adjusting member for adjusting said impedancecorresponding to changes in said condition, a follow-up impedanceincluding an impedance element and a member associated therewith foradjusting said impedance, a connection between said motor means and theadjustable member of said follow-up impedance for causing adjustment ofsaid follow-up impedance corresponding to changes in the position ofsaid control surface, means including said control and follow-upimpedances for controlling said motor means to move said surfaceproportionally to the movements of said condition responsive means,means for varying the ratio between a given movement of said conditionresponsive means and the following movement of said surface comprising avariable impedance connected in parallel with one of said previouslynamed impedance elements, and means responsive to the dynamic airpressure adjacent to said aircraft for operating said variableimpedance.

'7. Apparatus for controlling the position of a control surface on anaircraft comprising, in combination,l motor means for positioning saidsurface with respect to said aircraft, means movable in response to acondition indicative of the need for operation of said surface, acontrol impedance including an impedance element and a membercooperating therewith for adjusting the impedance value of saidimpedance, a connection between said condition responsive means and saidadjusting member for adjusting said impedance corresponding to changesin said condition, a follow-up impedance including an impedance elementand a member associated therewith for adjusting said impedance, aconnection between said motor means and the adjustable member of saidfollow-up impedance for causing adjustment of said follow-up impedancecorresponding to changes in the position of said control surface, meansincluding said control and follow-up impedances for controlling saidmotor means to move said surface proportionally to the movements of saidcondition responsive means, means for varying the ratio between a givenmovement of said condition responsive means and the following movementof said surface comprising a variable impedance comprising an impedanceelement connected in parallel with said first named impedance elementand a member cooperating therewith for adjusting the impedance value ofsaid impedance, said member and impedance element being so related thatthe rate of increase of the impedance value of said impedance increasesas said member is moved uniformly in one direction, and means responsiveto a condition indicative of the air resistance encountered by saidsurface for moving said last named member in said one direction uponsaid condition decreasing.

S. ISSERSTEDT.

